A process is described in United Kingdom 1,287,303, in which circulation of a buffered solution between a zone wherein hydroxylammonium salt is formed and a zone wherein cyclohexanone oxime is formed. The chemical reactions that take place in the different zones are represented by reaction equations 1-3, as follows.
1) Preparation of the hydroxylammonium salt: EQU 2 H.sub.3 PO.sub.4 +NO.sub.3.sup.- +3 H.sub.2 .fwdarw.NH.sub.3 OH.sup.+ +2 H.sub.2 PO.sub.4.sup.- +2 H.sub.2 O
2) Preparation of the oxime: EQU NH.sub.3 OH.sup.+ +2 H.sub.2 PO.sub.4.sup.- +2 H.sub.2 O+H.dbd.O.fwdarw.H.dbd.N--OH+H.sub.3 PO.sub.4 +H.sub.2 PO.sub.4.sup.- +3 H.sub.2 O
3) Supply of HNO.sub.3 to make up the depletion of the source of nitrate ions after removal of the oxime formed: EQU H.sub.2 PO.sub.4.sup.- +HNO.sub.3 .fwdarw.H.sub.3 PO.sub.4 +NO.sub.3.sup.-
The depletion of the nitrate ion source is made up by absorption into the circulating reaction mixture of gases containing nitrogen oxides, resulting in the formation of nitric acid. Directly supplying nitric acid to the reaction mixture is also possible. It is further possible to combine the absorption of nitrogen oxides and the supplying of nitric acid. When the nitric acid or the nitrogen oxides to be converted into nitrate have been added, the reaction mixture, has the same theoretical composition after the removal of water as the initial starting solution started from for the preparation of the hydroxylammonium salt. Furthermore, however, in the step involving the preparation of the hydroxylammonium salt, a quantity of ammonium ions is formed because the hydrogenation is not fully selective. In order to avoid an accumulation of ammonium ions in a continuous circulation process, the ammonium ions formed are converted by means of gases containing nitrogen oxides in accordance with the following reaction: EQU 2 NH.sub.4.sup.+ +NO+NO.sub.2 .fwdarw.2 N.sub.2 +3 H.sub.2 O+2 H.sup.+
This process is also referred to as the HPO (hydroxylamine phosphate oxime) process in the literature such as, Damme et al., Chemical Engineering, (Jul. 10, 1972), pages 54-55, the disclosure of which is incorporated herein by reference.
In the HPO process, the nitrogen oxides (nitrogen monoxide+nitrogen dioxide) are obtained by methods well known in the art by forced ammonia combustion, whereby ammonia and air are converted into nitrogen oxides and water. Part of the nitrogen monoxide formed is oxidized to nitrogen dioxide by means of secondary air, that is, air other than the stoichiometric amount of air. The HPO-ammonia combustion process is disadvantageous in that it has a small capacity, and therefore the efficiency of the ammonia combustion is lower than the efficiency of the ammonia combustion of a normal nitric acid production process, which is, in general, only 91% (on a per mol basis).
Another disadvantage is the great amount of process equipment which is needed for the nitric acid and nitrogen oxide preparation.
A further disadvantage is the environmental impact of the release of nitrogen oxides containing gases of a normal HPO process. It is quite expensive to build a nitrogen separation unit for such a small ammonia combustion. Thus, there is a need in the art to overcome this problem.